Andesite is an igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. Its mineral assembly is usually quartz and plagioclase. Biotite, hornblende and pyroxene are common accessory minerals. Alkali feldspar is absent.
Classification of andesites may be refined according to the most abundant phenocryst. Example: olivine andesite, if olivine is the principal accessory mineral. Andesite can be considered as the extrusive equivalent to plutonic diorite. As diorites, they are characteristic of subduction tectonic environments in active oceanic margins, such as the western coast of South America. The name andesite is derived from the Andes mountain range.
Andesitic magma in island arc regions (i.e. active oceanic margins) comes from wet melting of mantle wedge peridotite. When an oceanic plate is subducted, it contains a lot of water. This water is removed from the subducting slab because of the increasing pressure and temperature. Flow in the mantle wedge carries the water down to sites directly below the volcanoes, where the water enables the melting of the mantle peridotite. The initial melt is usually of basaltic composition. On its way to the surface, the melt stalls and cools, enabling the fractional crystallization of silica poor minerals, thus raising the silica content of the remaining melt and resulting in andesitic magma.

Andesite Line
The Andesite Line is the most significant regional distinction in the Pacific. It separates the deeper, basic igneous rock of the Central Pacific Basin from the partially submerged continental areas of acidic igneous rock on its margins. The Andesite Line follows the western edge of the islands off California and passes south of the Aleutian arc, along the eastern edge of the Kamchatka Peninsular, the Kuril Islands, Japan, the Mariana Islands, the Solomon Islands, and New Zealand. The dissimilarity continues northeastward along the western edge of the Albatross Cordillera along South America to Mexico, returning then to the islands off California. Indonesia, the Philippines, Japan, New Guinea, and New Zealand—all eastward extensions of the continental blocks of Australia and Asia—lie outside the Andesite Line.
Within the closed loop of the Andesite Line are most of the deep troughs, submerged volcanic mountains, and oceanic volcanic islands that characterize the Central Pacific Basin. It is here that basaltic lavas gently flow out of rifts to build huge dome-shaped volcanic mountains whose eroded summits form island arcs, chains, and clusters. Outside the Andesite Line, volcanism is of the explosive type, and the Pacific Ring of Fire is the world's foremost belt of explosive volcanism.
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